Structure of Small Blood Vessels PDF

Dr. Aly Shaalan Dr. Abeer Abdel Khalik  Lecture learning outcomes At the end of this lecture the student should be able to: 1. Describe the structure of arterioles. 2. Describe the structure of blood capillaries. 3. Describe the structure of postcapillary venules. ...

Dr. Aly Shaalan Dr. Abeer Abdel Khalik  Lecture learning outcomes At the end of this lecture the student should be able to: 1. Describe the structure of arterioles. 2. Describe the structure of blood capillaries. 3. Describe the structure of postcapillary venules.  Arterioles ✓Arterioles have three tunicae (layers) from inside to outside are intima, media and adventitia. 1.Tunica intima layer has endothelium only. 2.Tunica media has one or two layers of smooth muscle. 3.Tunica adventitia is thin layer of loose connective tissue. ✓Arterioles control blood flow to capillary networks by contraction of the smooth muscle cells.  Arteriovenous Shunts ✓Arterioles supplying a capillary bed typically form smaller branches called metarterioles in which the smooth muscle cells are dispersed as bands which act as precapillary sphincters which regulate blood flow into the capillaries.  Arteriovenous Shunts ✓The distal portion of the metarteriole, sometimes called a thoroughfare channel, lacks smooth muscle cells and merges with the postcapillary venule.  Arteriovenous Shunts ✓Branching from the metarteriole and thoroughfare channel are the smallest vessels (capillaries), which lack smooth muscle cells (although pericytes may be present).  Arterioles ✓Arterioles serve as flow regulators for the capillary beds. ✓In the normal relationship between an arteriole and a capillary network, contraction of the smooth muscle in the wall of an arteriole increases the vascular resistance and reduces or shuts off the blood going to the capillaries.  Capillaries ✓Capillaries are the smallest diameter blood vessels, often smaller than the diameter of an erythrocyte.  Capillaries ✓Capillaries form blood vascular networks that allow fluids containing gases, metabolites, and waste products to move through their thin walls.  Capillaries ✓Each capillary consists of a single layer of endothelial cells and their basal lamina. ✓The endothelial cells form a tube just large enough to allow the passage of red blood cells one at a time.  Capillaries ✓In many capillaries, the lumen is so narrow that the red cells literally fold on themselves to pass through the vessel.  Capillaries ✓Pericytes associated with capillaries represent a population of undifferentiated mesenchymal stem cells.  Capillaries Capillaries are classified into three different types: 1.Continuous (characterized by uninterrupted vascular endothelium and by the continuous basal lamina). 2.Fenestrated (characterized by numerous openings in the capillary wall and by the continuous basal lamina). 3.Discontinuous or sinusoidal capillaries (larger in diameter with large openings, intercellular gaps, and a discontinuous basal lamina).  Postcapillary Venules ✓Venules are the smallest of the veins. ✓Postcapillary are smaller than venules. ✓Postcapillary venules have pericytes instead of a tunica media, and their walls are more permeable than those of venules and even of capillaries. ✓They are responsible for the exchange of materials.  Postcapillary Venules ✓Vasodilator substances, such as serotonin and histamine, appear to act on small venules, causing them to become “leaky” by increasing the intercellular distances between the membranes of contiguous endothelial cells. ✓Most such intercellular gaps occur in postcapillary venules rather than in capillaries. ✓Leukocytes preferentially leave the vascular system at the postcapillary venules to enter the connective tissue spaces via diapedesis.  Arteriovenous Shunts ✓In certain tissues and organs, arterioles deviate from this simple path to accommodate various specialized functions. For example, thermoregulation by the skin involves arterioles that can bypass capillary networks and connect directly to venules.  Arteriovenous Shunts ✓The media and adventitia are thicker in these arteriovenous shunts (or arteriovenous anastomoses) and richly innervated by sympathetic and parasympathetic nerve fibers. THANK YOU